Density detection using real time discrete photon counting for fast moving targets

ABSTRACT

A system for detecting and graphically displaying a contents of a fast-moving target object comprises: a radiation source, having a position such that at least a portion of radiation emitted from the radiation source passes through the fast-moving target object, the fast-moving target object having a variable velocity and acceleration while maintaining a substantially constant distance from the radiation source and being selected from the group consisting of: a vehicle, a cargo container and a railroad car; a velocity measuring device configured to measure the variable velocity of the fast-moving target object; a detector array comprising a plurality of photon detectors, having a position such that at least some of the at least a portion of the radiation passing through the target object is received thereby, the detector array having a variable count time according to the variable velocity and a grid unit size; a counter circuit coupled to the detector array for discretely counting a number of photons entering individual photon detectors, the counter circuit measuring a count rate according to a contents within the fast-moving target object; a high baud-rate interface coupled to the counter circuit for sending count information from the counter circuit at a rate fast enough to support real-time data transfer therethrough; and a processor coupled to the velocity measuring device and to the high baud-rate interface, receiving count information from the high baud-rate interface and generating distortion-free image data in real time as a function of the count information and the variable velocity. A method for using the system is also disclosed.

This is a Continuation-in-Part Application of U.S. Ser. No. 08/921,854of Verbinski et al., for DENSITY DETECTION USING DISCRETE PHOTONCOUNTING, filed Sep. 2, 1997, now abandoned, which is a ContinuationApplication of U.S. Ser. No. 08/546,999 of Verbinski et al., for DENSITYDETECTION USING DISCRETE PHOTON COUNTING filed Oct. 23, 1995, nowabandoned, both of which are incorporated herein by reference.

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of this patentdocument or the patent disclosure, as it appears in the United StatesPatent and Trademark Office patent files or records, but otherwisereserves all copyright rights whatsoever.

COMPUTER PROGRAM LISTING APPENDIX

A computer program listing appendix is submitted herewith on a singlecompact disc. The computer program listing is incorporated-by-referenceherein its entirety. The compact disc is a CD-R disc labeled“09/398,547apx” and contains a single computer program listing that wassaved to CD-R on Aug. 6, 2002 and is 3.5 MB.

BACKGROUND OF THE INVENTION

The present invention relates to density detection using discrete photoncounting, and more particularly to using discrete photon counting togenerate an image indicative of the densities in a target object. Evenmore particularly, the invention relates to using discrete photoncounting in ultra high-speed real time detection, and distortion-freeimage processing for a fast-moving target, under acceleration.

There are many-instances in the security or customs field when it isnecessary to examine or inspect, in a non-destructive way, the contentsof a target object, such as a closed package, box, suitcase, cargocontainer, automobile semi-trailer, tanker truck, railroad car, e.g.,box car or tanker car, or the like. For example, customs departments areroutinely charged with the responsibility of inspecting vehicles cominginto a country to make sure such packages do not contain drugs or othercontraband, or leaving the country with stolen automobiles, drug money,and other illicit contraband. Similarly, drug smugglers frequently carryout their criminal acts by hiding illegal drugs in vehicles such tankertrucks, and then sending the trucks through a border checkpoint. Whensecurity personnel encounter suspicious vehicles or other containersbeing transported over international boundaries, they must perform acareful inspection of such vehicles to ascertain their contents.Similarly, when suspicious trucks or cars enter compounds overseashaving U.S. troops or containing embassy offices, they must be inspectedfor hidden vehicle bombs, poisonous gases, etc.

When suspicious vehicles are discovered, they generally must be examinedor inspected on location in what is referred to as a “secondaryinspection area.” If secondary inspection reveals the presence ofcontraband (e.g., drugs), then the vehicle may be impounded, the driverarrested, and the contraband disposed of. If, on the other hand, theexamination reveals the absence of contraband, then the vehicle may beallowed to proceed in a normal manner.

The process used to examine or inspect a suspicious vehicle should bequick, simple, as unintrusive as possible and fast enough so as to notimpede the “flow of commerce”. Unfortunately, most common conventionalinspection mechanisms require either visual inspection by others and/orscent inspection by dogs.

These conventional inspection methods require that the vehicle stop andwait for the inspection to be completed, which can take a half hour ormore. This is both inconvenient and time consuming for both customsofficials and the vehicle drivers and occupants. Furthermore, suchinspection may put officers at personal risk if a vehicle has beenbooby-trapped or if the vehicle's driver or other occupants becomenervous and decide to attack the customs officer inspecting theirvehicle. What is needed, therefore, is a non-invasive technique forinspecting the contents of a suspicious vehicle without requiring thatthe vehicle be stopped and manually inspected.

One attempt to satisfy this need involves the use of high levels ofradiation to determine the densities of the vehicle and/or the contentsof such vehicle. Unfortunately, this approach in the prior art requiresthat the vehicle be stopped and evacuated prior to inspection, becausesuch high levels of radiation can be physically harmful to the vehicle'soccupants if they remain in the vehicle during inspection.

Disadvantageously, prior art inspection systems using high levels ofradiation not only require that the vehicle be stopped, and thereforedelayed, but pose a risk to stowaways that may be aboard the vehicle,and unwilling to voluntarily evacuate when the vehicle is stopped forinspection. Therefore, what is needed is a non-invasive technique forinspecting the contents of a suspicious vehicle without requiring theuse of high levels of radiation.

A further problem posed by manual inspection techniques arises whentanker trucks or railroad cars, after having been emptied, seek to crossa border in order to refill. Because some such tankers (e.g., liquifiedpetroleum gas tankers that are of thick, double-walled steelconstruction) cannot be completely emptied without releasing thepressure in such tankers and venting noxious (and explosive) gasses intothe atmosphere, the tankers typically are kept nominally under pressure.(The venting of noxious gasses would be hazardous and ecologicallyunacceptable.) Thus, the contents of such tankers typically gouninspected by customs agents in order to avoid the time-consuming (upto 3 days, with nitrogen purging) venting of such gases. Unfortunately,drug smugglers are well aware of this fact, and therefore utilize tankertrucks and railroad cars to import illegal drugs, knowing that they willnot be inspected at the border. This venting condition provides just oneof numerous additional examples of cases where invasive or intrusiveinspection into vehicles, or other containers, is not feasible ordesirable. Thus, this venting condition further emphasizes the need fora non-intrusive approach to vehicle inspection, especially by ahigh-energy gamma-ray radiographic system that easily penetrates thesteel walled tanker.

Yet a further problem with prior vehicle inspection systems is thatsome, employing inspection sources, move a vehicle past a source anddetector with heavy equipment subject to frequent breakdowns, andrequiring very high capital costs for installation. Some inspect at arate as low as 10-15 minutes per cargo vehicle, according to U.S.Customs Inspectors.

Additionally, some prior systems employing a high intensity standardX-ray radiation source require from ½ hour to 1 hour to warm up,depending upon the intervals between use. The X-ray source is expensiveto buy and to install and requires an appreciable amount of power tooperate, is sensitive to ambient humidity, and is expensive andtime-consuming to repair.

Furthermore, these expensive X-ray sources also require a permanentshielding structure, which, along with the vehicle-moving mechanism,boosts the capital costs to nearly $10,000,000 for one such system,limiting the numbers which can be in use at borders.

Therefore, there is a widely known need in the industry of cargo-vehicleinspection systems for a mobile, vehicle inspection system capable ofdetecting contraband on the order of less than a pound (or better) in alarge, fast-moving vehicle, with the use of relatively very lowintensity radiation (on the order of Curie or better), in a manner whichcan be done swiftly so as not to hold up vehicle traffic at borderinspection points, and affordably, even with a fast-moving, large,accelerating vehicle, accelerating at an unpredictable rate.

The present invention advantageously addresses the above and otherneeds.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as wellas other needs by providing a system and method employing discretephoton counting, and a relatively very low intensity radiation source,to perform ultra high-speed real-time density measurements in afast-moving, target object and to generate a distortion-free, highresolution image of contents of such fast-moving (and accelerating),target object in response thereto.

In one embodiment, the invention is characterized as a system fordetecting and graphically displaying contents of a fast-moving targetobject comprising: a radiation source, having a position such that atleast a portion of radiation emitted from the radiation source passesthrough the fast-moving target object, the fast-moving target objecthaving a variable velocity and acceleration while maintaining asubstantially constant distance from the radiation source and beingselected from the group consisting of: a vehicle, a cargo container, anda railroad car; a velocity measuring device configured to measure thevariable velocity of the fast-moving target object; a detector arraycomprising a plurality of photon detectors, having a position such thatat least some of the at least a portion of the radiation passing throughthe fast-moving target object is received thereby, the detector arrayhaving a variable count time according to the variable velocity and agrid unit size; a counter circuit coupled to the detector array fordiscretely counting a number of photons entering individual photondetectors of the detector array, the counter circuit measuring a countrate according to a contents within the fast-moving target object; ahigh baud rate interface coupled to the counter circuit for sendingcount information from the counter circuit at a rate fast enough tosupport real-time data transfer therethrough; and a processor coupled tothe velocity measuring device and to the high baud-rate interface,receiving count information from the high baud-rate interface andgenerating distortion-free image data in real time as a function of thecount information and the variable velocity.

In another embodiment a method of detecting and graphically displayingwithout distortion, in real time, a contents of a fast-moving targetobject comprises the steps of: directing photons from a radiation sourcetoward the fast-moving target object having a variable velocity andacceleration while maintaining a substantially constant distance fromthe radiation source, and being selected from the group consisting of: avehicle, a cargo container, and a railroad car; receiving at least aportion of the photons directed at the fast-moving target object at adetector array maintaining substantial equidistance from the radiationsource; measuring the variable velocity and acceleration of thefast-moving target with a velocity measuring device coupled to aprocessor; discretely counting a number of the photons entering each ofa plurality of detectors in the detector array with a counter circuit ata count rate according to the contents; sending count data on the numberof photons from the counter circuit to the processor through a highbaud-rate interface at a processing rate fast enough to supportreal-time data transfer from the counter circuit; receiving the countdata at a graphical display device from the processor; and generatingdistortion-free image data in real time, as a function of the count dataand the variable velocity of the contents of the fast-moving targetobject.

In a variation of the above method, a method of detecting a density,without distortion, within a fast-moving target object, includesperforming the above steps for radiation passing through a first andsecond volume of the fast-moving target object, hitting a first andsecond radiation detector.

In a further embodiment, a method of vehicle inspection employs a mobileplatform in a moving target mode, to detect and graphically display, inreal time, without distortion, a contents of a fast-moving targetobject. The method comprises the steps of: positioning a mobile platformhaving a boom and a detector array tower thereon; wherein the boom has aradiation source, and wherein the detector array tower houses a detectorarray; deploying the boom to a distance and configuration relative tothe mobile platform and the detector array tower sufficient to allowpassage of a vehicle therethrough; accelerating, from a stationaryposition, the fast-moving target object between the detector array towerand the radiation source so as to maintain a substantially equaldistance from the radiation source; directing photons from the radiationsource toward the fast-moving target object upon acceleration of thefast-moving target object; receiving at least a portion of the photonsdirected at the fast-moving target object at the detector array;measuring the variable velocity and acceleration of the fast-movingtarget with a velocity measuring device coupled to a processor;discretely counting a number of the photons entering each of a pluralityof detectors in the detector array with a counter circuit measuring acount rate according to the contents; sending count data on the numberof photons from the counter to the processor through a high baud-rateinterface in real-time data; receiving the count data at a graphicaldisplay device from the processor; and generating distortion-free imagedata in real time as a function of the count data and the variablevelocity and acceleration of the contents.

In a variation of the above embodiment, a mobile platform can be used ina stationary target mode, by accelerating the mobile platform andperforming analogous steps.

In yet a further embodiment, the invention can be characterized as alinear detector array system for use in a vehicle inspection system fordetecting a contents of a fast-moving target object. The linear detectorarray comprises: a plurality of vertical rows of staggered detectors,each of the plurality of vertical rows being vertically staggered from,yet not overshadowing, each other vertical row, such that a pitchbetween any two closest adjacent staggered detectors is smaller than adiameter of the staggered detectors.

In yet a further embodiment, a method for processing staggered detectiondata for use in a vehicle inspection system, comprises the steps of:providing a plurality of vertical rows of staggered detectors, each ofthe plurality of vertical rows being vertically plurality of verticalrows of staggered detectors, each of the plurality of vertical rowsbeing vertically staggered from, yet not overshadowing, each othervertical row, such that a pitch between any two closest adjacentstaggered detectors is smaller than a diameter of the staggereddetectors; including: providing a center vertical row of staggereddetectors; providing one or more side vertical rows of staggereddetectors; providing a processor comprising an image-generating program;receiving data at the processor from each of the one or more sidevertical rows and from the center vertical row; determining anadjustment for a horizontal displacement k of the one or more sidevertical rows using the adjustment to coincide data from the sidevertical rows with data from the center vertical row. In this embodimentthe three (or any n number of) rows of detectors are used rather than asingle row of appreciably smaller detectors, so that a higher detectorcount rate can be achieved. This improves inspection speeds at a givensource strength.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a schematic diagram of a system made in accordance with oneembodiment of the present invention and of a tanker truck containingcontraband material, wherein discrete photon counting is used to performdensity measurements on a tanker truck in conjunction with crosscorrelation means for velocity measuring, wherein a velocity-compensatedimage is generated of the contents of such tanker truck in responsethereto;

FIG. 1A is an alternate detector configuration to a single rowconfiguration using three vertical rows of staggered detectors toachieve a smaller pitch than would otherwise be possible in a verticallinear detector, for the same detector size (and count rate) and whichmay optionally be employed in the system of FIG. 1 for an increasedimage resolution;

FIG. 2 is a schematic diagram of a system made in accordance withanother embodiment of the present invention and of a tanker truckcontaining contraband material, wherein discrete photon counting is usedto perform density measurements in a tanker truck and wherein an imageis generated of the contents of such tanker truck in response thereto;

FIG. 3 is a schematic diagram of a system, for inspecting a long trainof freight cars, made in accordance with yet another embodiment of thepresent invention including a magnetic pick-off system (wheel transducerunit) for measuring velocity of a fast-moving target (freight car);

FIG. 3A is a perspective view of a mobile uniplatformed vehicleinspection system employing the detector configuration of FIG. 1A inaccordance with a further embodiment of the present invention;

FIG. 4 is a block diagram of the system of FIGS. 1, 2 and 3 showinggamma/x-ray detectors coupled through 16-channel processing units,accumulators, RS-485 line drivers, and an RS-485 interface card to acomputer, wherein the computer processes discrete photon countinformation and target velocity from detectors and a velocity-measuringdevice and causes a display device to display an image of contents of afast-moving target object, such as the tanker truck of FIGS. 1 and 2, inresponse thereto;

FIG. 5 is a block diagram showing the detectors of FIGS. 1 and 2 coupledthrough preamplifiers, amplifiers, discriminators, accumulators, and anRS-485 line driver that make up one embodiment of the 16-channelprocessing units of FIG. 4;

FIGS. 6A, 6B, 6C, 6D, 6E and 6F are schematic diagrams showing onevariation of an analog portion the 16-channel processing units of FIG.4;

FIGS. 7A and 7B are schematic diagrams showing one variation of adigital portion of the 16-channel processing units of FIG. 4;

FIG. 8 is a block diagram of functional components that make up oneembodiment of a software system with which the computer of FIGS. 1 and 2is controlled;

FIG. 9 is a flow chart showing the steps traversed by the computer ofFIGS. 1 and 2 in response to the software system of FIG. 7 when an imagegeneration program is executed; and

FIG. 10 is a diagram illustrating a preferred screen layout for theimage displayed on the display device of FIGS. 1 and,2.

APPENDIX A is a source code listing of a firmware operating systemincluding steps traversed by each 16-channel processing unit of FIG. 4in order to quickly relay the photon count information received from thedetectors to the computer.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles of theinvention. The scope of the invention should be determined withreference to the claims.

Referring first to FIG. 1, a schematic diagram is shown of a system madein accordance with one embodiment of the present invention and of afast-moving target object 10 (“target object”, or “tanker truck”,“truck”, or “railroad car”) containing contraband, wherein discretephoton counting is used with one of several possible velocity-measuringmeans, a cross-correlation velocity measuring means, to measure density;

an image is generated of measured density in response thereto.

Shown in FIG. 1 are the fast-moving target object 10, concealedcontraband 12, a detector array 14, a radiation source 18, electronics(or “computer electronics”) 22, a graphical display device (display) 24,a video camera 1010, a video interface 1020, and a cross-correlationcomputer 1015.

In an alternative configuration illustrated in FIG. 2, and describedlater herein, the radiation source 18 and the detector array 14 areuniformly able to move with respect to the fast-moving target object 10.

By comparison, in the configuration of FIG. 1, the fast-moving targetobject 10 (or truck or railroad car) being inspected can be drivenbetween the detector array 14 and the radiation source 18 at highwayspeeds of up to about 60 miles per hour or more. In the illustratedembodiment of FIG. 1, the detector array 14 and the radiation source 18are both stationarily mounted.

The detector array 14 employs a plurality of “oversized” high efficiencygamma-ray detectors 26, e.g., up to three hundred and thirty-six (336),detectors arranged in a vertical column. The detectors 26 make itpossible to scan the fast-moving target object (tanker truck or railroadcar) 10 with a very low intensity gamma-ray field. In order tofacilitate the use of very low intensity gamma-radiation, the oversizeddetectors 26 are used, such as are available as Part No. 1.5M1.5M1.5,NaI (Tl) (sodium iodide crystal, thallium activated) (with R2060photomultiplier tube) from BICRON of Ohio. Such gamma-ray detectors arescintillation counter-type detectors and are 1.5″ in diameter, 2.5″high, and mounted on a 1.5″ photo-multiplier tube (PMT).

Optionally, an analogous detector of around 0.5 to 1.5″ in diameter maybe used to obtain a finer grid unit mapping of 0.4 to 1.0 inches alongone or more dimensions of the fast-moving target object 10.

Alternatively, detectors having a pitch P (i.e., space from one detectorcenter to a next closest detector center) of 0.4 to 1.0″ are used toscan a smaller grid unit along the fast-moving target object 10 to apixel, using a similar configuration of the target object 10, theradiation source 18 and the detector array 14.

Optionally, a pitch P of smaller than twice a radius of the detectors 26is achieved by staggering detectors vertically such that their circularsurfaces lie in a single plane, thereby avoiding any shadowing ofdetectors by other detectors, and then compensating for the staggeringby computer computations as described in detail later herein.

The very low intensity gamma-ray field useable with gamma-ray detectors26 is low enough in intensity to allow operating personnel to workwithin it, when a fast-opening shutter (“shutter”; not shown) of theradiation source 18 is closed. In the illustrated embodiment the shutteris opened only when an image is being generated, preferably after allpersonnel leave an area swept out by a fan beam of the radiation source18.

For example, the very low intensity gamma-ray field may use 662 keVgamma-ray energy from a Cs-137 radiation source. However, a strongergamma-ray or x-ray source than this can be used, in the interest offaster density measurements, while still allowing operating personnel tosafely work within the very low intensity field. The 662 KeV gamma-rayenergy can be used, however, when the vehicle under inspection istraveling at high-speeds e.g., railroad freight car or highway speeds,and when the shutter is opened, e.g., after a truck driver or trainengineer has passed.

The embodiments of the invention described herein expose the cargo toonly about 5 microroentgen of gamma radiation or about 15 minutes worthof natural background radiation.

Preferably the radiation source 18 is, in one 15 configuration, a 1, 1.6or 2.0 Curie shuttered mono-energetic source of Cs-137 gamma-rays (662keV gamma-ray energy).

Alternatively, a nearly mono-energetic Co-60 source may be used whichemits photons at 2 energy levels, in particular, 1170 and 1339 keV. Amono-energetic or near mono-energetic source is preferable, however,because energy-level filtering of the “softer component” (as in X-rays)can be eliminated. A suitable source is readily available as Model No.SH-F2 from Ohmart Corporation of Ohio. The radiation source is used incombination with a collimator that provides a 45° vertical opening(measured from horizontal upwards) and a 10° lateral opening. Thefast-opening shutter is electrically actuated.

The radiation source 18 provides gamma-rays that are of high enoughenergy levels (e.g., 662 KeV) to be penetrating of steel walls and onlymoderately attenuated by steel walls typically found in tanker trucks orrailroad cars. Yet such rays are sufficiently attenuated by contrabandpackages to make them easily detectable by measuring the penetration ofthe gamma-rays emitted from the source and deriving relative materialdensities therefrom. In addition, there is negligible backscattering ofthe gamma-ray energy from the tanker walls, and, in any case, much lessthan would occur if a high-powered x-ray source was utilized. (Although,a highly filtered x-ray source could, in other embodiments, be employedfor high-speed inspection applications or for inspection of unmannedvehicles, such a highly filtered source, adds costs and complexity tothe system, and detracts from reliability. For these reasons, it is notpreferred.)

Referring still to FIG. 1, a velocity measuring system using across-correlation method, employs the video camera 1010 coupled to thecross-correlation computer 1015 through the video interface 1020, tomeasure a velocity v of the fast-moving target object 10 at thecross-correlation computer 1015, using photographic images taken attimes T₁ and T₂, by the video camera 1010. The cross-correlationcomputer 1015 sends velocity information to the electronics 22(including a computer). The video camera 1010, located a distance D fromthe fast-moving target object 10 (e.g., a tanker truck), takes a firstphotographic image (a “frame” or “first image”) at T₁; it then takes asecond photographic image (“second image”) at T₂. Times T₁ and T₂correspond to a difference in time ΔT. The fast-moving target object 10moves a distance Δd in the time ΔT. The Computer 1015 calculates a ratio$\frac{\left( {\Delta \quad d} \right)}{\left( {\Delta \quad T} \right)}$

equal to a velocity, v, of the fast-moving target object 10 and utilizesthe varying value of v to determine a count time per grid unit (or“mapped pixel unit” or “mapped pixel size”) at time T₁ through T₂ toproduce an undistorted image (i.e., where a square is not imaged as ashortened or lengthened rectangle).

To obtain Δd the second image at T₂ is moved until it overlaps with thefirst image at T₁. The distance moved, Δd, to obtain a best overlap, israpidly calculated by performing a Fast Fourier analysis on each of thefirst and the second image in digital format before and after achievinga best overlap. A resulting cross-correlation function yields thedistance Δd the fast-moving target object 10 moved between time T₁ andT₂ The values of Δd and Δt then determine the velocity, v, of thefast-moving target object 10 during a time span between T₁ and T₂. Thevelocity, v, is effectively an average velocity during the time span.

A count time (also “count” or “sample time”) T_(c) at each of thedetectors 26 is selected as a function of a fixed distance Δx of travelof the fast-moving target object 10, and of the measured velocity, v, ofthe fast-moving target object 10.

Preferably, to avoid distortion, a value of the fixed distance Δx is thefixed distance horizontal grid unit size and is selected, for example,to be equal to Δy, a vertical grid unit size, wherein Δy is selectedaccording to center to center detector spacing, Pitch, P, wherein Δy=Z/DP, Z=source-target distance, and D=source-detector distance. By settingthe value of the fixed distance Δx equal to the vertical grid unit sizeΔy, an undistorted gamma-ray radiography-like image results, regardlessof the velocity of the fast-moving target object 10.

A distortion-free image is generated with pixels (not shown) on thedisplay 24, representing an area of Δx by Δy within the fast-movingtarget object 10 in real time, line by vertical line, as the fast-movingtarget object 10 passes between the detector array 14 and the radiationsource 18. While the fast-moving target object 10 is in motion, thevelocity of the fast-moving target object 10 is either assumed to beconstant, or is measured continuously, in which case the count timeT_(c) varies as frequently as each vertical line of pixels. In the latercase, the velocity at each instant, is read into the image-generatingcomputer 36 to adjust the count time T_(c) to the fixed distance Δx,corresponding to the horizontal measure of each horizontal grid unit,(“picture element”, or “mapped pixel”) during each sample time T_(c).

Since each grid unit correlated to a pixel, the value of Δx is set equalto Δy, and Δy is proportional to a spacing between neighboring detectors26, a radiographic-like image in real time, vertical line by verticalline, during relative motion between the fast-moving target object 10,the source 18 and the detector array 14, is achieved without distortion,despite variations in the velocity and acceleration of the fast-movingtarget object 10 as it passes between the detector array 14 and theradiation source 18.

This image is generated in real-time at high velocities by fastdata-processing circuitry 30; drivers 32 and interface 34. For eachdetector 26, a count rate (or “count”) per detector 26 is measuredrepresentative of a number of photons passing through the grid unithitting the detector 26 wherein the count rate is high enough to achievea statistically accurate measure of a contents or density being sampledin a given grid unit (Δx by Δy), as the fast-moving target object 10moves a distance Δx at high-speed. The count rate gives a measure of thedensity or contents of the fast-moving target object 10, by means of therelationship ln (N_(o)/N)=(dls), where N_(o) is a detector count rate inair (calibration constant), N is a count rate for a pixel correspondingto a target material of density d, a thickness l, and a cross-section s,with gamma rays passing through a target area (Δx by Δy).

The count rate is further achieved by a fast analog pulse amplifier 42,(described later herein) electrically coupled to a photomultiplier-tubetype of detector 26 with NaI scintillator, that can operate at a rate ofup to two (2) million counts per second. A high-speed discriminator 44,(described later herein) also operational at the count rate, biasedabove electronic noise, generates a pulse for each gamma ray detected.The pulses are then counted in an accumulator circuit, 47, accessed eachcount time T_(c), (wherein T_(c) equals Δx/v).

For example, with 64 detectors 26 spaced at a pitch P of 2.5 inchesapart, 36 feet from the radiation source 18 (D=36 feet) and thefast-moving target object 10 at 25 feet from the radiation source 18(Z=25) then$\Delta \quad y\quad {equals}\quad 2.5\quad {(25)/36}\left( {= \frac{P\quad Z}{D}} \right)\quad {equals}\quad 1.76$

inches. Thus, for this configuration, the fixed distance Δx is 1.76inches. For the fast-moving target object 10 traveling at about 60miles/hour (966 kilometers/hour), this results in a sample time T_(c) ofabout 1.7 milliseconds, meaning that the detectors 26 are sampled, at afrequency of about 600 times per second.

Thus, the vertical “linear array” configuration of the detectors 26 ismade to provide a resolution of grid points spaced about every 1.76inches along the length of the target vehicle, and about 1.76 inches, onaverage, along the height of the target vehicle (as projected on thetarget vehicle vertical lengthwise center plane) when the vehicle isclose to a detector tower.

This resolution is adequate to achieve a detectability limit of aslittle as about one pound of contraband per 1.76 inches by 1.76 inchesgridpoint (or mapped pixel unit).

By definition, vertical grid unit or vertical scanning length along thetarget object (“vertical grid unit” or “vertical resolution”) Δy isequal to $\quad {{\frac{Z}{D} \cdot P};}$

(as employed above) wherein Z=the distance from the radiation source 18to a center of the fast-moving target object 10; D=distance from thedetector array 14 to the radiation source 18; and P, pitch=verticaldistance from a center of a detector 26 to another center of a nextclosest detector 26.

In accordance with one variation, the grid unit size (corresponding to apixel) can easily be varied by appropriately selecting the location ofthe vehicle with respect to the radiation source and the detectors 26within the detector array 14, and by varying the distance betweeninspection points, Δx, longitudinally (via choice of sampling periodalong the length of the target vehicle). For example, in the aboveexample employing a 2.5 inch pitch, if a vehicle is half way between theradiation source 18 and the detector array 14, the vertical resolutionis 2.5/2=1.25 inches, and the value of Δx is set to 1.25 inches as well.

Spacing between the detectors in the detector array can be varied, orfor example, counts from adjacent pairs of detectors in the detectorarray can be combined, to change the mapped pixel size Δy in thevertical direction.

A smaller grid unit of 0.4 inches or less may be scanned to a pixel byusing a pitch, P, of 0.4 to 1.0 inches while holding the value of$\frac{Z}{D}$

from between 1 and 0.4. For example, in an embodiment illustrated byFIG. 1A, three (3) rows of staggered detectors are employed to achieve apitch smaller than a diameter of the staggered photon detectors 202. Acomputer corrects for horizontal displacements of two rows of the threerows of staggered detectors.

Referring next to FIG. 1A, a plurality of staggered photon detectors 202are employed wherein a low level intensity radiation source (e.g. 1.6Curies) may optionally be employed in accordance herewith. The staggereddetectors 202 may preferably be oversized (e.g. about 1.5″ diameter andabout 2.5″ long) and have a pitch P smaller than the diameter 2r of thestaggered detectors 202.

Three (3) vertical rows R of staggered detectors 202 are employed,instead of a single row of detectors 26 shown in FIG. 1. The three (3)vertical rows R are vertically staggered from each other. The pitch Pbetween two (2) closest adjacent such staggered detectors 204, 206 maypreferably be about 0.7″, when employing staggered detectors 202 havinga 1.5″ diameter, thereby yielding a count rate of about 20,000counts/second for each staggered detector 202. This pitch P results in avertical resolution, R or vertical grid unit of about 0.411, when theradiation source 18 is a distance D of 20′ from the staggered detector202 and the radiation source 18 is a distance z of 11½′ from a center ofthe fast-moving target object 10 wherein R_(vert)=Pz/D.

The staggered detectors 202 are staggered from each other in a verticaldirection, yet their circular surfaces all lie in a same plane, therebyavoiding shadowing from any other staggered detector 202 while enablinga smaller pitch P.

The image-generating program corrects for horizontal displacement ofeach of two (2) side rows S of the staggered detectors 202 from a centerrow C (of the 3 vertical rows R) of staggered detectors 202 in thefollowing manner.

A center vertical-line image is first generated for the center verticalrow C. The image-generating software then superposes on the center rowvertical line image, other side images corresponding to each of the two(2) side vertical rows S by moving each of the two (2) side images adistance l=kx/D (referred to the center of the fast-moving target 10) ina horizontal direction to coincide with the central-row image, wherein kis a horizon distance between each of the vertical rows R of thestaggered detectors 202, and x and D have been previously defined as,respectively, the distance between the radiation source 18 and thecenter of the fast-moving target object 10. and the distance between theradiation source 18 and the staggered detectors 202.

For example, employing two (2) vertical rows R of the staggereddetectors 202, each of 2¼″ diameter, the following advantageous resultsare achievable: (1) the vertical resolution, R_(vert) (vertical gridunit) size is selected to be around 1.0″ utilizing staggered detectors202 with a diameter of 2¼″, and (2) the count rate of the staggereddetectors may optionally be about 90,000 photon counts/second with the1.0″ vertical resolution, which is high enough to achieve a relativelyhigh speed photon imaging capability for the 1.0″ vertical resolution.This count rate is adequate for high speed scanning, yielding about 1000counts per grid unit at 5 miles per hour and 500 counts per grid unit(pixel) at 10 miles per hour scanning speed.

In general, a scanning speed is proportional to a square of a grid unitsize. For example, if a 1″ grid unit size is increased to a 2″ grid unitsize, employing the same number of counts/pixel, the scanning speed maybe increased by a factor of four (4), since the scanning speed isincreased by the square of a ratio of the grid unit sizes (i.e. 2″/1″).

Preferably, an entire length of the fast-moving target 10 is scannedautomatically with a fan beam, in a single sweep. For example, an entiretrain of about 100 to 200 freight cars, traveling at up to 10 mph, canbe inspected at Laredo, Texas, as the train enters the United States. Atthese inspection speeds, the “flow of commerce” is not impeded.

Referring next to FIG. 2, an analogous configuration is used for analternate arrangement employing a radiation source 18 and a detectorarray 14, wherein the radiation source 18 and the detector array 14 aremoving synchronously in respect to a stationary target object 10. Shownin FIG. 2 are the stationary target object (truck) 10, concealedcontraband 12, a detector array 14, a detector array truck or trolley16, a radiation source 18, a radiation source truck or trolley 20,processing electronics 22, a graphical display device 24, and a computer36.

In this embodiment, the detector array truck 16 and the radiation sourcetruck 20 are designed to travel synchronously along parallel tracks. Thetrucks 16, 20, are mounted on tracks, and employ a synchronous drivemotor (not shown) and a variable frequency generator (not shown) forcontrolling the speed of the synchronous drive motor, such as areavailable as Model No. SA0100 from Becker Equipment (Mark Becker P. E.)of Vista, Calif. However, numerous known substitutes can be employedtherefore.

In operation, the trucks 16, 20 are moved synchronously along parallelpaths spanning the entire length of the target object 10 to beinspected.

In FIG. 2 automatic scanning in the truck-mounted embodiment shown isaccomplished when the detector array truck 16 and the radiation sourcetruck 20 move in a parallel fashion along the tracks at a constantspeed, with a counting interval selected to effect a longitudinal gridunit size (i.e., grid-spacing interval) of 0.4 to 1 or 1 to 2 inches fora typical tanker truck inspection.

This grid unit size, as mentioned above, can easily be selected by oneskilled in the art based on the disclosure provided herein and dependentupon an optimum tradeoff between minimum contraband contentdetectability, throughput (i.e., inspection time per tanker truck), andgamma-ray field-strength (and other safety concerns).

Thus, in the preferred embodiments shown in FIGS. 1, 1A, and FIG. 2, atruly non-invasive inspection technique is provided in which there is noneed to manually inspect the vehicle, or, with the fast-opening shutter,to even stop or slow the truck 10. The shutter is closed after thetarget object 10 (truck or entire train) passes by. With such rapidinspection capability, the flow of commerce is relatively unimpeded,even with 100% inspection of cargo vehicles.

Referring next to FIG. 3, one embodiment of a system made in accordancewith yet another embodiment of the present invention including avelocity-measuring system is shown. A similar velocity-measuring systemcould be employed in the system of FIG. 1 in lieu of the system shown inFIG. 1.

In the case of a train crossing an international border, wherein suchtrain cannot be adequately velocity-controlled for obtainingdistortion-free images, a velocity-measuring system is extremelyadvantageous.

In the system shown in FIG. 3, which is especially well suited forrailway train applications, a magnetic pick-off system can be employed.The magnetic pick off system illustrated in FIG. 3 includes a pair ofvelocity sensors (or wheel transducer units) 310, spaced a knowndistance apart to determine the velocity v of the train 300 at eachinstant after detecting the train's passage. As a wheel 380 of the train300 passes each of the pair of wheel transducer units 310, a time isclocked and recorded. A known distance (e.g., inches) and a differencein time is enough to compute the velocity of the train 300. The measuredvelocity v is calculated by an Auxiliary Processor Unit, 340, coupled toa modem 350. The modem 350 sends a velocity signal through an RS-232line 360 into a host computer 370 coupled to the RS-232 line 360.

Image software contained within the host computer 370 is then used tocompute a detector sampling period or the count time T_(c) so that acontents of the train 300 corresponding to a fixed grid unit size Δx andΔy defined earlier herein is detected by the detector array 14 andmapped to a pixel in an undistorted fashion. In this fashion, anundistorted image is achieved independent of the velocity v of the train300 passing between the detector array 14 and the radiation source 18during each sampling period T_(c).

Another velocity-measuring system, a doppler radar system (not shown)such as the Railroad Falcon, developed for Science ApplicationsInternational Corporation (SAIC), of San Diego, Calif. by the FALCONCorporation, measures velocities from 0.3 miles per hour to 99 miles perhour with a precision of +0.1 miles per hour. This is an alternativemethod of measuring the velocity v of the truck or the train 300 beinginspected. The doppler radar system is similar to a police-type radargun used for interdiction of speeders along the highway, except it isspecially engineered to yield high precision, and to measure down to thevery low-velocity limit of trains crossing the border or trucksaccelerating from a stand-still. The Railroad Falcon has an RS-232output for reading the velocity signal into the computer 1015 forgenerating an image.

A radar range finder (not shown) with high precision and providing manyrange readings per second, may also be employed in a further variationof the system, such as a mobile vehicle inspection system such asillustrated in FIG. 3A.

In yet a further variation of the invention system particularly suitedfor trucks entering an inspection station, the velocity-measuring devicemay be a commercially available device that utilizes a pressure pad thatis activated when a truck tire passes over the pressure pad. Employingtwo or more such pressure pads, spaced at a known distance apart,provides a measure of the velocity of the vehicle that can be fed to theprocessing electronics 22 (FIGS. 1 and 2) as a part of the imagegeneration process.

However, regardless of the velocity information is generated andregardless of the particular embodiment of the system employed, thevelocity information is fed into the processing electronics 22 (FIGS. 1and 2) to determine the sampling period or count time T for thedetectors 26, so as to obtain a fixed horizontal grid unit size, Δx,that matches the vertical grid unit size, Δy determined by the pitch,the vertical spacing between detectors, and the proximity of the train300 or the fast-moving target 10 to the radiation source.

Thus, employing any of the variations of the velocity-measuring system,if a sufficiently high field-strength (or field intensity) is utilized,the detector array 14 (FIGS. 1 and 2) and the radiation source 18 may befixed or stationary rather than mounted on the radiation source truck(or trolley) 20 and the detector array truck (or trolley) 16 illustratedin FIG. 2. In such an arrangement, the tanker truck 10 or railroad carcan be driven past the detector array 14 and radiation source 18 withthe determination as to the densities within the truck 10 being madeautomatically by adjusting the time interval of detector readings inorder to normalize the horizontal pixel width Δx as the truck 10 orrailroad car passes between the radiation source 14 and the detectorarray 18.

In a variation of the above-embodiment, the fast-opening shutter(“shutter” not shown) adds further protection of an occupant of thefast-moving target object 10, the train 300 or the truck 10 of FIG. 2.In accordance with this variation of the invention, the shutter, placedat the radiation source 18, in a line-of-sight to the detector array 14,remains closed when an occupant passes through the line-of-sight. Whenclosed, the shutter blocks gamma rays from leaving the radiation source18, providing heightened safety by not exposing the occupant to theradiation.

The shutter opens very quickly (e.g., in 250 milliseconds) after anengine passes the radiation source 18, if the engine is in front of thefast-moving target object 10, the train 300 or truck 10. The shuttercloses before the engine passes the radiation source 18, if the engineis in back of the fast-moving target object 10. For added safety, thisshutter closes by return-spring action in event of an electrical powerfailure.

Referring next to FIG. 3A, a mobile, uniplatformed, vehicle inspectionsystem (mobile system) 300′ is shown wherein both a radiation source 18′and a linear detector array 14′ are mounted on only one mobile platform,such as a truck, and are deployed using a controllable source boom(source boom) 310′ to effect the proper spacing for passage of afast-moving or stationary target 10 therebetween.

The mobile system (“mobile system”) 300′ comprises a truck 16′; theradiation source 18′ suspended at the end of the controllable sourceboom 310′ that is coupled to the truck 16′; and the linear detectorarray 14′ also coupled to the truck 16′. The source boom 310′ is longenough such that when it is deployed, the radiation source 18′ and thelinear detector array 14′, are sufficiently laterally spaced so as toallow for the passage of the fast-moving target 10 therethrough.

The mobile system 300′ is optionally used in two possible modes ofoperation, 1) a stationary-target mode, and 2) a fast-moving (or moving)target mode. The radiation source 18′, such as a 1.6 Ci Cs-137 source,is suspended from a far end of the source boom 310′ so as to facilitateimaging of the fast-moving target object 10 in either of the twopossible modes. The radiation source 18′ is opened during scanning ofthe fast-moving target object 10, and a narrow fan-shaped beam isdirected at the linear detector 14′.

In one configuration, the linear detector 14′ is a 15′ high detectorarray including five (5) three-foot modules. Each three-foot modulecomprises three (3) vertical rows of 1.5 inch diameter, 2.5 inch longNaI (TI) detectors, with sixteen (16) detectors 26 in each vertical row.The three (3) rows are staggered vertically, such as illustrated by FIG.1A, such that the staggered detectors 202 of 1.5 inch diameter, providepitch , P, of about 0.72 inches, and vertical resolution about 0.48inches. Then, an image-generating computer (not shown) such as the hostcomputer 370 generates an image in the manner such as described for thedetector configuration illustrated in FIG. 1A.

In the stationary-target mode, the truck 16′ scans the target object 10while the target object is stationary and without an occupant, while thetruck 16′ moves along a length of the fast-moving target object 10 toproduce a full image of its contents. Advantageously, the truck 16′ neednot move at exactly the same speed during the entire scan because thetime constant T_(c) between which detector readings (photon counts) arerecorded is varied as a function of the velocity of the truck (which ismonitored by the image-generating computer, which receives a velocitysignal from speedometer equipment aboard the truck), in order tomaintain a substantially constant horizontal pixel width Δx.

Optionally, in the alternate moving target mode, the truck 16′ isstationary and the occupant of the target object 10 drives the targetobject 10 just past a source fan beam region 320′ to avoid theradiation. The shutter (not shown), such as described earlier, is thenopened and the occupant drives the fast moving target object 10 at abouta nominal rate of acceleration which has been clocked at about 33inches/sec².

While the fast-moving target object 10 is accelerating, a velocitymeasuring system (such as one of the velocity-measuring systemsdescribed hereinabove), such as shown and described in reference to FIG.1, or such as a high repetition conventional radar range as mentionedearlier herein, is aimed at the target object 10 and measures positiondata thereof several times per second. The position data is sent to animage-generating computer (not shown), such as the host computer 370shown in FIG. 3.

The position data, together with time data, is next converted intovelocity data by the image-generating computer (not shown) to form avelocity profile v(t). Simultaneous with the acceleration of the targetobject 10, the image-generating computer starts to generate an image ofthe target object 10 in real-time, by setting a count time T_(c) (asdefined earlier) for each detector equal to a time required for thefast-moving target object 10 to move the fixed distance of thehorizontal grid unit size, Δx (Δt=Δx/v) described earlier, wherein Δx ispreferably set to equal the vertical grid unit size Δy and wherein Δy isproportional to the detector pitch P, previously defined ascenter-to-center vertical distance between neighboring detectors 26 orstaggered detectors 202. The proportionality of Δy to pitch P has beenpreviously described herein.

The moving target mode of operation requires as little as about 6seconds to fully image the fast-moving target object 10 for anaccelerating vehicle. In the stationary target mode of operation, thefast-moving target object 10 can be inspected at about 5 miles/hour orgreater, while the mobile system 300′ maintains the horizontal and thevertical resolution (grid unit) for imaging of about 0.5 inches, inaccordance herewith.

Employing any of the above-cited velocity measuring systems enables themobile system 300′ to scan and image at a variety of variable speeds andaccelerations while still maintaining excellent imaging resolution, anddistortion-free images, at ultra high speeds (relative to heretoforeknown imaging approaches) such as up to about 60 miles per hour.

Advantageously, therefore a velocity of the target object 10 or a mobilesystem can be selected and adapted according to the mission at hand. Asthe target velocity increases, for a similar configuration of the source18, 18′ and the detector array 14, 14′, a color or grey-scale tonedefinition per pixel, or the number of colors which that pixel can have,effectively decreases accordingly. This decrease in color definition perpixel occurs because as the target velocity increases, a number ofphotons reaching the detector 26 in the detector array 14 decreases,since a count time T_(c) is decreased, as it takes less time for atarget length corresponding to one pixel to pass the detector 26. Sincethere are less overall photons to count, (a smaller number ofcounts/pixel), the counts can be distributed among fewer colors orgrey-scale tones than if there were a higher count rate.

Accordingly, if a high throughput is required, a higher target or mobilesystem velocity may be selected, sacrificing some color definition asdescribed above. Otherwise, if a higher color definition image isrequired, such as for disarming an explosive device, a lower target ormobile system velocity may be selected.

Furthermore, in cases involving stolen vehicle detection, where highthroughput or speed is of the highest importance and image resolution isnot as important, and where there are three (3) rows of detectors 202,such as shown in FIG. 6, two (2) of the three (3) rows of detectors 202may be ignored for imaging (thereby reducing image resolution, i.e.,increasing grid unit size of a pixel) in the interest of speed orprocessing time.

Referring next to FIG. 4, a block diagram is shown of the systems ofFIGS. 1 and 2 showing gamma/x-ray detectors coupled through 16-channelprocessing units, accumulators, RS-485 line drivers, and an RS-485interface card to a computer, wherein the computer processes discretephoton count information received from the detectors 26 and causes adisplay device to display an image of the contents of a target object10, such as the tanker truck of FIG. 1, or FIG. 2 in response thereto.

The detector array 14 is depicted in FIGS. 1 and 2, as are theelectronics 22 and the graphical display device 24. The detector array14 employs the plurality of gamma/x-ray detectors 26. The gamma/x-raydetectors 26 are coupled in groups of 16 gamma/x-ray detectors each toaccumulators, which are in-turn coupled to 16-channel data processingcircuits 30. In practice, the number of gamma/x-ray detectors 26 useddepends on the height of the vehicles to be inspected and the desiredresolution, i.e., number of pixels, in the image desired.

In one embodiment, especially favorable for detecting car-sized objectswithin a vehicle, a cargo container, or a railroad car, 48 gamma/x-raydetectors are employed in a linear vertical fashion and a grid unit sizeor resolution of about 2.5 inches is selected.

In another embodiment, especially favorable for faster-moving targets,the detector array 14 comprises 64 detectors 26 with a pitch of 1.76″,the detectors 26 being sampled at 600 times per second (sampling every1.7 msec per detector) corresponding to a speed of 60 mph of thefast-moving target object 10.

In another variation, especially favorable for finer spatial resolution,referred to as VACIS-II, three (3) vertical rows of 112 detectors 26each, (336 detectors 26) are employed and a vertical and horizontalresolution of about 0.4 inches is selected (or 0.42 inches).

The 16-channel data processing circuits 30, each include an accumulator47, the 16-channel data processing circuits 30 being coupled to anRS-485 line driver/firmware (“driver/firmware”) 32, which is coupled toan RS-485 interface (or RS-485 card) 34. The RS-485 interface 34 isembodied on a circuit card located within a computer system 36. Asuitable RS-485 interface is available as Model No. 516-485, Part No.3054 from Sea Level Systems, Inc., and from numerous other vendors underrespective model/part number designations.

The computer system 36, which is preferably a Pentium-300 based personalcomputer system, or a faster (newer) computer system, operatesprogrammatically under the control of a software system.

The computer system 36 receives data on velocity from a velocitymeasuring device 35, such as any of the devices described herein (seeFIG. 1, FIG. 3), and uses the velocity data to adjust the count timeτ_(c) as previously defined herein (as a sample period for the detectors26).

The computer system 36 also receives information on an accumulatedphoton count from the accumulator 47 through the driver/firmware 32(described later) originating initially from detector pulses from eachof the 16-channel data processors 30, in response to the detection ofindividual photons by the gamma/x-ray detectors 26, 202 (FIGS. 1, 1A and2). As explained in further detail herein below, the software systemaccepts a value of the accumulated photons counts passed to it by adiscriminator 44, which ensures each pulse height, from energy depositedin the detector by the photons, is above an electronic noise level.Advantageously, the accumulated photon counts permits for a noiselesssignal, as compared to measuring current from many more photons whichhas associated current noise, because each photon is counted above anoise threshold. The software system generates a radiographic image-likedisplay output signal in response to the accumulated counts.

The radiographic, image-like display output signal generated by thecomposite software is coupled to the graphical display device 38, whichis preferably a Super-VGA monitor, and is used by the graphical displaydevice 38 to generate a graphical representation of the densities withinthe vehicle under inspection.

Unlike some prior art systems, which do not generate a graphicalrepresentation, i.e., a “picture”, of the densities of the contents ofthe vehicle under inspection, the present embodiment generates such apicture.

In addition, unlike prior art systems, in this particular embodiment,each vertical line composing this picture is generated sequentially inreal time, while the fast-moving target changes position relative to thesource 18 and to the detector array 14.

Advantageously, this allows for easy, instantaneous, direct visualinterpretation of the results of the scanning of the vehicle underinspection, making possible prompt interdiction of the vehicle beforeunloading the contraband, as opposed to interpreting more subtleindications of the densities within the vehicle under inspection as maybe required in prior art systems.

Advantageously, the preferred software system also causes the display ofa reference image simultaneous with the image generated in response tothe vehicle under inspection, so that an operator of the presentembodiment can easily make a visual comparison between what a vehicle ofthe type being inspected should “look like”, and what the vehicle underinspection actually “looks like”. Such “side-by-side” inspection furthersimplifies the detection of contraband using the present embodiment.

As a result of the very low intensity gamma-ray or X-ray radiation usedby the present embodiment, photon penetration, as opposed tobackscatter, can be used to generate a side, as opposed to a bottom/top,image of the vehicle under inspection, because a radiation exclusionzone is small for a low-intensity field. This represents a significantimprovement over prior art systems wherein a bottom/top presentation ofthe radiation source is required to avoid the need for excessiveradiation shielding, but dictates that the vehicle's frame, drive train,wheels, etc., interfere with the density measurements taken based onradiation penetration. Backscatter-type density measurement systems areless accurate due to the non-uniform backscattered radiation on whichthey rely for density measurement. In addition, back-scattered photonshave significantly decreased energy, and are less penetrating and cannoteffectively measure high pressure tanker trucks with double-walled thicksteel walls.

Referring next to FIG. 5, a block diagram is shown of the detectors ofFIG. 4 coupled through preamplifiers 40, amplifiers 42, thresholddiscriminators (discriminators) 44, accumulators 47, and an RS-485 linedriver/firmware 32, that make up one embodiment of the 16-channelprocessing units of FIG. 4. The RS-485 driver/firmware 32 comprisesmicroprocessor firmware 31, a communications controller 33 and a linedriver 39.

Each of the radiation detectors 26 is coupled to a preamplifier 40within the 16-channel data processing unit 30. Each preamplifier 40 iscoupled to an amplifier 42, which is in turn coupled to a discriminator44. Each discriminator 44 generates an electrical pulse for each photondetected above an electronic noise level by the radiation detector 26coupled thereto.

Advantageously, the use of (non-integrating) discrete photon counting atthe levels of photon fluxes employed herewith (i.e. relatively very lowradiation intensity) together with the use of the discriminator 44 toallow photons (or pulses) to be counted only above a cut-off thresholdenergy, allows for a much improved, virtually noiseless system, using alower strength sources than conventionally used.

In accordance herewith, every pulse generated from the discriminator 44represents an actual photon from the radiation source 18, 18′; that is,only photons generated from the radiation source 18, 18′ are counted atthe accumulators 47. Since each pulse counted at the accumulator 47represents an actual photon from the radiation source 18, 18′, aphoton-by-photon count (or count rate) at the accumulator 47 representsa virtually noiseless signal of how many photons from the radiationsource 18, 18′ hit each detector 26.

This photon-by-photon virtually noiseless signal has several advantagesover some prior arts systems wherein photon-integration over a myriad ofphoton energy levels is employed. Even with such prior art, or state ofthe art integrating discrete photon counting systems (such as may beused with standard X-ray detectors) it is necessary to bombard thedetector 26 with many more orders of magnitudes of photons in order todrown out a substantial noise contribution (e.g., leakage current) of asignal.

Problematically, because so many more orders of magnitudes of photonsare necessary to obtain the signal with an integrating discrete photondetector, a common method of integrating a detector count (or countrate) is to generate a current from charge collected as a result ofenergy being deposited in a crystal of the detector 26, rather than tocount the pulses generated for each photon deposited in the detector 26.The strength of the current is then measured, in the conventionalsystems, instead of the total number of photon counts in accordance withthe present invention. From this current must be subtracted a varying(temperature dependent) background current.

Another further problem with this form of integration, is that there isalways some parametric leakage current involved in a circuit or a solidstate device measuring the current, and this further contributes to thenoise of the signal, worsening the initial problem.

With the prior integrating discrete photon counting systems, not onlymore photons are needed, but also a much higher source strength and amuch longer 10 inspection time is needed in order to generate enoughphotons necessary to do the integration or to generate the current fromthe induced charge created by the energy deposition of the photons. Thisis also problematic because a higher source strength means higher dosesof radiation, and additional power and expense.

Therefore, preferably, a mono-energetic source such as a 662 keV γ-raysource of Cs-137 or a near mono-energetic source such as Co-60 (dualenergy, one energy level at 1170 keV and another at 1330 keV) isemployed to make discrete photon counting at the threshold cut-offenergy level or narrowband much easier, since the radiation source 18 isconstant with time and need not be filtered to filter out a soft, lowerenergy component.

Alternatively, in accordance herewith, any source could be used inconjunction with a filter placed around the radiation-source 18, 18′,filtering out photons of energies outside a desired energy level.

Because very low intensity gamma-ray or x-ray radiation is used with thepresent embodiment, pulse pileup is generally not of significantconcern. Count rates of up to 90,000 counts/seconds are presently beingcounted with negligible “chance coincidence” loss with pulse amplifierscapable of counting nearly two million counts/seconds (via 40 nanosecondamplifier “pulse” time constants).

The discriminators 44 within each of the 16-channel data processingunits 30 are coupled through the accumulator 47 to a linedriver/firmware (RS-485 driver/firmware) 32 which includes amicroprocessor firmware (processor) 31 coupled at an output to acommunications controller 33 coupled at an output to a line driver 39.Each of the 16-channel data processing units 30 includes its own linedriver/firmware 32. The line drivers/firmware (RS-485 primer/firmware)32 operate under the programmatic control of a firmware operating systemin the processor 31 which processor 31 also controls the communicationscontroller 33 and the line driver 39, such as shown in APPENDIX A.

In operation, the preamplifiers 40, and amplifiers 42 function in aconventional manner to amplify signals generated by the detectors 26connected thereto. Outputs of the amplifiers 42 are passed along to thediscriminators 44, which accept pulses that are well above a noisethreshold (e.g, about twice the noise threshold). The pulses are passedby the discriminator 44 and then counted in an accumulator 47 for eachdetector 26, resulting in accumulated counted pulses (accumulated pulsecounts), thereby generating a count rate as previously discussed.

The line driver/firmware 32 passes the accumulated pulse counts, whichpulses are accepted past a threshold by each of the discriminators 44and passed to each accumulator 47, within a particular 16-channel dataprocessing unit 30, along to the computer via the RS-485 interface 34illustrated in FIG. 4.

Referring next to FIGS. 6A, 6B, 6C, 6D, 6E and 6F, schematic diagramsare shown of one variation of an analog portion the 16-channelprocessing units of FIGS. 4 and 5. The schematics of FIGS. 6A, 6B, 6C,6D, 6E and 6F are self-explanatory to one of skill in the art ofcircuits and therefore further explanation of these figures is not madeherein.

Referring next to FIGS. 7A and 7B, schematic diagrams are shown of onevariation of a digital portion of the 16-channel processing units ofFIGS. 5 and 6. The schematics of FIGS. 7A and 7B are self-explanatory toone of skill in the art and therefore further explanation of thesefigures is not made herein.

Referring next to FIG. 8, a block diagram is shown of functionalcomponents that make up one embodiment of a software system with whichthe host computer 370 of FIG. 4 is controlled.

Upon initialization (Block 100), the computer, under control of thesoftware system, initializes (Block 102), and loads a default color mapdisplay (Block 104), which maps detected densities within the vehicleunder inspection, i.e., pulse counts, to specific colors to be producedon the display device 38 (shown in FIG. 4). Next, the user is presentedwith a main menu (Block 106), and the computer is instructed to waituntil an operator instructs the software system as to what step to takenext.

One of the options available to the operator is a help function (Block108). The help function displays tutorial and/or reference informationto the operator via the display device, as is common in the computingarts.

Another option presented to the operator is the “Display Image from Diskin Upper Window” option (Block 110). When selected, this option allowsthe operator to load a saved display image from a hard or floppy diskdrive within the computer, and to automatically display the image in theupper display window on the display drive. (See FIG. 10) Generally, theupper display window, in accordance with the present embodiment, is usedto display a reference image, i.e., an image of the same make of truckunder inspection, but while empty, i.e., containing no contraband.

A further option that can be selected by the operator is a “DisplayImage from Disk in Lower Window” option (Block 112). When selected thisoption allows the operator to load a saved image from a hard or floppydisk drive within the computer, and automatically displays the image inthe lower display window on the display drive. (See FIG. 10) Generally,the lower display window, in accordance with the present embodiment, isused to display an inspection image, i.e., an image of the vehicle underinspection. A useful function of this option is for reinspection of avehicle at a later time by a supervisor in order to maintain qualitycontrol. Because the image is stored on disk, it is not necessary thatthe vehicle be present when this re-inspection takes place. The savedimage of the vehicle, after being loaded, can easily be visuallycompared with the reference image loaded into the upper display window.

The next option available to the operator is the “Save Image from LowerWindow to Disk” option. This option can be used to save an image of avehicle under inspection for later reinspection, or can be used to savea reference image after a known empty vehicle has been inspected, i.e.,scanned using the present embodiment.

Using a “Load Color Lookup Table from Disk” option (Block 116), theoperator is able to load a previously saved color lookup table fromdisk. This allows the user to retrieve a color map, different from thedefault color map, so that a different set of colors can be mapped tovarious density measurements, i.e., pulse counts within the vehicle.

The next option is the “Acquire Image from Counters and Display toScreen” option (Block 118). This option initiates an image generationprogram, as described below in reference to FIG. 9, which causes thedetector array 14 and the radiation source 18 to perform densitymeasurements and causes the display of an image indicative of thevarious densities within the vehicle under inspection in the lowerdisplay window on the screen display. Advantageously, the presentembodiment allows the operator to display a reference image in the upperdisplay window while the inspection is being conducted, so that he orshe can visually compare what the vehicle under inspection should looklike empty with what the vehicle under inspection in fact looks like. Inthis way, the operator is able to determine whether or not the vehicleunder inspection may contain contraband.

The next two options (Blocks 120 and 122) allow the operator to setvalues for what is referred to herein as the “K” constraint and the “L”constraint. These two “constraints” function in a manner similar to thewell known functioning of the brightness and contrast controls oncommonly available cathode ray tube-type displays. These values affectthe mapping of colors to the various pulse counts, which is performed asfollows:

(1) a “white” level, i.e., a number of counts corresponding to zerodensity, is determined for each sensor during detector calibration,which is a step in image acquisition as described below in reference toFIG. 7;

(2) the variable “T” is then set equal to this white level times thereciprocal of the number of photons counted by a particular detector ata particular horizontal position on the vehicle;

(3) if “T” less than one, i.e., more photons are counted than the whitelevel, then T is set equal to one;

(4) the variable “D” is then computed as follows:

 D=254/(1+L)ln(T*K),

 where L and K are the constraints mentioned above, which are initiallyset to one, and where T is defined above; and

(5) if “D” is less than 1 or greater than 254, then D is set to 1 or254, respectively.

The significance of the number 254 in the above computations is thatthere are 256 possible colors displayable on the preferred Super-VGAdisplay device, however this number could be adjusted up or down toyield an appropriate color mapping where more or fewer colors aredisplayable.

Other options available to the user are options to “Redisplay BothWindows” (Block 124), “Redisplay the Upper Window” (Block 126) and“Redisplay the Lower Window” (Block 128). Redisplay options such asthese are useful to the operator if the images displayed on the displaydevice 38 become corrupted in some way, as for example may occur if textis sent to the display device 38 while it is displaying a graphicalimage.

The user may also “Reset a Default Color Map Array” (Block 130), “Load aNext Color Map Array” (Block 132) and “Load a Previous Color Map Array”(Block 134). These options are used to step through variouspreconfigured color maps, and to reestablish the default color map, sothat the operator can utilize a color map that best emphasizes thefeatures of the vehicle under inspection that he or she is inspecting.

Other options available to the user are to “Reset Modified Color Table”(Block 136), “Increase Color Table Gain” (Block 138), “Decrease ColorTable Gain” (Block 140), “Increase Color Table Offset” (Block 142), and“Decrease Color Table Offset” (Block 144). These options affect the“mx+b” relationship between the photon counts determined by thegamma/x-ray detectors and the colors displayed on the display device.The “gain” (m) is initially set, or can be reset, to one, and the“offset” (b) is initially set, or can be reset, to zero. These twoparameters allow the operator to “zoom” in on a particular range ofdensities for mapping within the color table by increasing or decreasingthe offset in order to establish a minimum density of interest (withevery density below this density being mapped to zero density (or“white”), and by increasing or decreasing the gain in order to establisha maximum density of interest (with every density above this densitybeing mapped to maximum density (or “black”).

A final operator-selectable option depicted in FIG. 8 is an “End” option(Block 146). This option is used by the operator to exit the softwaresystem and to return control to an operating system, such as is known inthe art of computer technology.

Referring next to FIG. 9, a flow chart is shown of the steps traversedby the computer 36 of FIG. 4 in response to the software system of FIG.8 when an image generation program is executed.

Upon being initiated (Block 200), the image generation is initialized(Block 202), and the movement of either 1) the radiation source truck20, and the detector array truck 16, if used, or alternatively, 2) themovement of the mobile system 300′ in another embodiment, or 3) themovement of the target object 10, is initiated (Block 204). Next, thedetectors 26 are calibrated (Block 206) by irradiating the detectorswith the radiation source 18 at a point along the track before theradiation source 18, 18′ and the detector array 14, 14′ become alignedwith the vehicle or the target object 10, to be inspected, such that ahorizontal length of the target object will be traversed uponcontinuation of the initiated movement, e.g., before the vehicle underinspection is interposed between the detector array 14, 14′, and theradiation source 18, 18′. Such irradiation of the detectors 26establishes a baseline of radiation (or “white” photon count level)corresponding to a density in the vehicle being inspected ofapproximately zero density and corresponding to a maximum photon count.Three photon counts are made in this manner for each detector 26. Suchcounts are then arranged for each detector 26 and then stored in anarray having a white level element for each detector 26.

A horizontal position is then set to zero (Block 208). The horizontalposition corresponds to a position along the track or a mobile systempath or a target object path arbitrarily selected to be a first positionat which density measurements are taken. Irrespective of whichembodiment is employed or which reference is moving (thedetector-source, or the vehicle or target object 10), this horizontalposition should be at a point before the vehicle is interposed betweenthe detector array 14 and the radiation source 18.

Next, a detector count is set to zero (Block 210), which corresponds toa first of the detectors 26 in the detector array 14 to be queried for aphoton count. If the target object 10 is moving, a velocity of thetarget object 10 is measured by any of the several methods describedearlier, herein, and a count time per grid unit is set (Block 211)according to the measured variable velocity of the target object toeffect a desired mapped grid unit size without distortion. Next, thisdetector is queried for a photon count and is instructed to restartcounting photons (Block 212). In response to this instruction, thedetector queried restarts counting photons (Block 214) and thepreviously determined number of photon counts is passed along to thecomputer (Block 216). This number of photon counts is stored into anarray within a memory in the computer (Block 218) and is then convertedinto a pixel value (Block 220). Conversion into the pixel value includesmapping the number of photon counts to a color to be displayed on thedisplay device. Such mapping is described more completely above inreference to FIG. 8.

Next, the detector number queried is converted into a vertical positionon the screen display (Block 222) and the horizontal position of theradiation source 18, 18′ or the mobile system path or the target objectpath and the detector array 14, 14′ along the tracks is converted to ahorizontal position on the screen display. Next, the pixel at thedetermined horizontal and vertical positions is illuminated using thecolor corresponding to the number of photon counts, as previouslyconverted (Block 224).

Next, a determination is made as to whether all of the detectors 26 inthe detector array 14 have been queried for a number of photon countsfor the current horizontal position (Block 226). If all the detectorshave not been queried (Block 226), the detector number to be queried isincremented (Block 227) and execution of the image generation programcontinues by querying the next detector in the detector array 14 for thenumber of photon counts, and by instructing such detector to restartcounting (Block 212). Execution continues from this point as describedabove (Block 214 et seq.)

If all the detectors 26 within the detector array 14 have been queriedfor the current horizontal position (Block 226), the horizontal positionis incremented (Block 228) and a determination is made as to whether ornot the target object 10 or the radiation source 18, 18′ and thedetector array 14, 14′ are still moving (Block 230). If the radiationsource 18, 18′ and 35 the detector array 14, 14′ are still moving (Block230), the detector to be queried is reset to zero (Block 210) andexecution of the image generation program continues as described above(Block 212 et seq.).

If the target object 10 or the radiation source 18, 18′ and the detectorarray 14, 14′ have stopped moving (e.g., because they have reached thefarthest point of travel down the tracks or the mobile system path orthe target object path (Block 230)), execution of the image generationprogram is terminated (Block 232).

Referring next to FIG. 10, a diagram is shown illustrating a preferredscreen layout for the images displayed on the display device of FIG. 4.

As shown, the screen display 300 is divided into an upper display 302, alower display 304 and a color bar 306. In accordance with the presentembodiment, the upper display 302 can be used, as mentioned above, todisplay images stored on disk. These images will generally be referenceimages used for visual comparison with an image representative of avehicle under inspection. The lower display 304, in addition to beingable to display images stored on disk, is used to display images, asthey are generated, indicative of the various densities within thevehicle under inspection. Both the upper and lower displays 302, 304 arecolor mapped using the current color map, gain and offset, so that theycan be visually compared to one another.

Any differences in a reference image, and an image generated duringinspection of a vehicle may indicate the presence of contraband withinthe vehicle under inspection.

The color bar 306 indicates the colors that are mapped to the variousdensities detectable by the present embodiment, serving as a referenceto the operator as to which colors indicate higher densities thanothers. As suggested in FIG. 10, colors nearer to the top of the colorbar 306 are indicative of more density, i.e., fewer photons counted aspenetrating the vehicle under inspection, and colors nearer to thebottom of the color bar 306 are indicative of less density, i.e., morephotons counted as penetrating the vehicle under inspection.

Thus, a system and associated methods are provided in the presentembodiment for determining the densities within a vehicle underinspection based on discrete photon counting, and for generating animage indicative of such densities. Advantageously, such determinationis made based on discrete photon counting, thereby eliminating the needfor high levels of gamma-ray or x-ray radiation.

The present embodiment, thus, eliminates the need to stop and manuallyinspect vehicles at border crossings, and other inspection points. Inaddition, the present embodiment, because of the very low levels orgamma-ray or x-ray radiation, advantageously eliminates the need to stopand evacuate the vehicle before it is subjected to very high strengthgamma-ray or x-ray radiation, when the radiation source shutter opensjust after the driver has passed: The scattered radiation dosage to thedriver is very low, and of an acceptably minute level. Advantageously,one variation the present embodiment provides for the determination ofdensities within the vehicle without the need even to stop the vehicle,such as a train. Slightly higher radiation levels may, in accordancewith this variation, be used to reduce or even eliminate the slowingneeded to determine the densities within a vehicle, and to generate animage indicative thereof, if the radiation source is closed when thedriver is “in the beam.”

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. A system for detecting and graphically displayinga contents of a fast-moving target object comprising: a radiationsource, having a position such that at least a portion of radiationemitted from the radiation source passes through the fast-moving targetobject, the fast-moving target object having a variable velocity andacceleration while maintaining a substantially constant distance fromthe radiation source and being selected from the group consisting of: avehicle, a cargo container, and a railroad car; a velocity measuringdevice configured to measure the variable velocity of the fast-movingtarget object; a detector array comprising a plurality of photondetectors, having a position such that at least some of the at least aportion of the radiation passing through the target object is receivedthereby, the detector array having a variable count time according tothe variable velocity and a grid unit size; a counter circuit coupled tothe detector array for discretely counting a number of photons enteringindividual photon detectors of the detector array, the counter circuitmeasuring a count rate according to a contents within the fast-movingtarget object; a high baud rate interface coupled to the counter circuitfor sending count information from the counter circuit at a rate fastenough to support real-time data transfer therethrough; and a processorcoupled to the velocity measuring device and to the high baud-rateinterface, receiving count information from the high baud-rate interfaceand generating distortion-free image data in real time as a function ofthe count information and the variable velocity.
 2. The system of claim1 wherein the radiation source is selected from the group consisting of:a gamma radiation source and a filtered x-ray source.
 3. The system ofclaim 1 wherein the radiation source is about a 662 KeV radiationsource.
 4. The system of claim 1 wherein the radiation source is amono-energetic radiation source.
 5. The system of claim 1 wherein theradiation source is a near mono-energetic radiation source.
 6. Thesystem of claim 1 wherein the radiation source is a Cesium-137 source.7. The system of claim 1 wherein the radiation source is a Cobalt-60source.
 8. The system of claim 1 wherein the radiation source is asource of photons with high enough energy to penetrate through thefast-moving target object.
 9. The system of claim 1 wherein theradiation source is a source of photons with high enough energy topenetrate through double steel walls of a conventional tanker truck. 10.The system of claim 1 wherein the radiation source is a relatively verylow strength radiation source emitting a fan beam of relatively very lowphoton intensity for purposes of scanning the fast-moving target object.11. The system of claim 1 wherein the radiation source is a Cesium-137source having a strength ranging from about 0.3 to 2 Curies.
 12. Thesystem of claim 1 wherein said plurality of photon detectors is arrangedin a linear column.
 13. The system of claim 1 wherein the detector arraycomprises forty eight (48) photon detectors and wherein each of theforty eight (48) photon detectors is configured to support a grid unitsize of about 2.5 inches.
 14. The system of claim 1 wherein the detectorarray comprises sixty four (64) photon detectors and wherein each of thesixty four (64) photon detectors has a diameter of about 1.5 inches anda pitch of about 1.8 inches.
 15. The system of claim 1 wherein thedetector array comprises sixty four (64) photon detectors and whereineach of the sixty four (64) photon detectors has a diameter of about 1.5inches and is sampled every 1.7 msec.
 16. The system of claim 1 whereinthe detector array comprises three hundred and thirty six (336) photondetectors each configured to support a grid unit size of about 0.4. 17.The system of claim 1 wherein the detector array comprises a pluralityof vertical rows of photon detectors, each of the plurality of verticalrows being vertically staggered from, yet not overshadowing, each othervertical row.
 18. The system of claim 17 wherein a pitch between any twoclosest vertically staggered detectors in adjacent vertical rows issmaller than a diameter of the staggered detectors to increaseresolution of the detector array.
 19. The system of claim 18 wherein thepitch is from 0.4 inch to 1.0 inch.
 20. The system of claim 1 whereinthe count rate measured by the counter circuit is as high as two millionphotons per second.
 21. The system of claim 1 further comprising agraphical display device and a storage device for storing a graphicaldisplay of the distortion-free image data at time t.
 22. The system ofclaim 21 further including a retrieval device for retrieving a storeddisplay and a display device for simultaneously displaying the storeddisplay and another graphical display of the distortion-free image datafrom another time.
 23. The system of claim 1 wherein the grid unit sizeis an area equal to a horizontal grid unit size multiplied by a verticalgrid unit size, and wherein the horizontal grid unit size is set equalto the vertical grid unit size to avoid image distortion.
 24. The systemof claim 23 wherein the vertical grid unit size is proportional to avertical pitch between each of the plurality of photon detectors. 25.The system of claim 23 wherein the horizontal and the vertical grid unitsize is set to correspond to detectability of contents weighing aslittle as about a pound.
 26. The system of claim 1 wherein the high baudrate interface sends count information in real-time when the fast-movingtarget object moves at about sixty (60) miles per hour and the grid unitsize is about 1.7 inches.
 27. The system of claim 1 wherein thefast-moving target object has a velocity of at least about 60 miles perhour.
 28. The system of claim 1 further comprising a collimator coupledto the radiation source having a vertical opening and a horizontalopening.
 29. The system of claim 28 wherein the vertical opening isabout 45 degrees.
 30. The system of claim 28 wherein the horizontalopening is about 10 degrees.
 31. The system of claim 1 furthercomprising a shutter coupled to the radiation source for blocking gammarays from leaving the radiation source to avoid exposing an occupant ofthe fast-moving target object to radiation.
 32. The system of claim 1wherein the radiation source exposes a cargo of the target object toonly about 5 microroentgen of gamma radiation equivalent to about 15minutes of natural background radiation.
 33. The system of claim 1wherein the detector arrays has a resolution of about 1.76 inches inlength and about 1.76 inches in height for achieving a detectabilitylimit of about one pound of contraband.
 34. The system of claim 1wherein the processor includes means for forming a distortion-free,radiographic-like image in real time during relative motion between thefast-moving target object and the detector array wherein the relativemotion includes variations in velocity and acceleration.
 35. A method ofdetecting and graphically displaying without distortion, in real time, acontents of a fast-moving target object comprising the steps of:directing photons from a radiation source toward the fast-moving targetobject, the fast-moving target object having a variable velocity andacceleration while maintaining a substantially constant distance fromthe radiation source and selected from the group consisting of: avehicle, a cargo container, and a railroad car; receiving at least aportion of the photons directed at the fast-moving target object at adetector array which maintains substantial equidistance from theradiation source; measuring the variable velocity and acceleration ofthe fast-moving target with a velocity measuring device coupled to aprocessor; discretely counting a number of the photons entering each ofa plurality of detectors in the detector array with a counter circuit ata count rate according to the contents; sending count data on the numberof photons from the counter circuit to the processor through a highbaud-rate interface at a processing rate fast enough to supportreal-time data transfer from the counter circuit; receiving the countdata at a graphical display device from the processor; and generatingdistortion-free image data in real time as a function of the count dataand the variable velocity of the contents of the fast-moving targetobject.
 36. The method of claim 35 wherein the step directing photonsincludes directing photons in a fan beam from a relatively very lowintensity radiation source for purposes of scanning the fast-movingobject.
 37. The method of claim 35 wherein the step of directing photonsincludes directing photons in a fan beam from a source having a strengthfrom about 0.3 to 2 Curies.
 38. The method of claim 35 wherein the stepof directing photons includes directing gamma photons in a fan beam froma gamma ray radiation source.
 39. The method of claim 35 wherein thestep of directing photons includes directing photons in a fan beam froma Cesium-137 radiation source.
 40. The method of claim 35 wherein thestep of directing photons includes directing photons in a fan beam froma Cobalt-60 radiation source.
 41. The method of claim 35 wherein thestep of directing photons includes directing photons in a fan beam froma mono-energetic radiation source.
 42. The method of claim 35 whereinthe step of directing photons includes directing photons in a fan beamfrom a near mono-energetic radiation source.
 43. The method of claim 35wherein the step of directing photons includes directing 662 keV photonsin a fan beam from the radiation source.
 44. The method of claim 35wherein the step of directing photons includes directing photons in afan beam from the radiation source wherein the photons have enoughenergy to penetrate through the fast-moving target object.
 45. Themethod of claim 35 wherein the step of directing photons includesdirecting photons in a fan beam from the radiation source wherein thephotons have enough energy to penetrate through double steel walls of aconventional tanker truck.
 46. The method of claim 35 wherein the stepof sending count data on the number of photons from the counter circuitis performed at a rate fast enough to support real-time data transferfrom the counter circuit when the variable velocity is about sixty (60)miles per hour and the grid unit size is about 1.7 inches.
 47. Themethod of claim 35 wherein the step of sending count data on the numberof photons from the counter circuit is performed about every 1700microseconds.
 48. The method of claim 35 wherein the step of discretelycounting a number of photons includes discretely counting at the countrate of as high as two million photons per second per photon detector.49. The method of claim 35 wherein measuring the variable velocitycomprises cross-correlating a first photographic image taken at a firsttime with a second photographic image taken at a second time todetermine the variable velocity between the first time and the secondtime.
 50. The method of claim 35 wherein receiving the count datacomprises selecting a count time at each of the plurality of detectorsin the detector array as a function of a fixed distance of travel of thefast-moving target.
 51. The method of claim 50 wherein the fixeddistance is selected to be equal to a vertical grid unit size.
 52. Themethod of claim 50 wherein selecting the count time comprises adjustingthe count time to the fixed distance corresponding to the variablevelocity during a sample time.
 53. A method of detecting a density,without distortion, within a fast-moving target object, the methodincluding: irradiating the fast-moving target object with a radiationsource, the fast-moving target object having a variable velocity andacceleration while maintaining a substantially constant distance fromthe radiation source and selected from the group consisting of: avehicle, a cargo container, and a railroad car; detecting with a lineardetector array a first discrete number of photons penetrating thefast-moving target object through a first prescribed volume and enteringa first radiation detector; detecting a second discrete number ofphotons penetrating the fast-moving target object through a secondprescribed volume and entering a second radiation detector, the firstand second detectors; measuring the variable velocity and a accelerationof the fast-moving target with a velocity measuring device coupled to aprocessor, discretely counting with a counter circuit coupled to thedetector array, a first number of photons detected by the firstradiation detector, the counter circuit measuring a count rate accordingto a first density; discretely counting with the counter circuit asecond number of photons detected by the second radiation detector, thecounter circuit measuring a second count rate according to a seconddensity; sending the first and second number of photons from the countercircuit to the processor through a high baud rate interface coupled tothe counter circuit in real-time; generating distortion-free image datain real time as a function of the first number and the second number ofphotons and the variable velocity and acceleration; and displaying, inreal time, a graphical display of the first density within the firstvolume of the fast-moving target object and the second density withinthe second volume of the fast-moving target at a graphical displaydevice coupled to the processor.
 54. The method of claim 53 wherein saidgenerating includes: mapping said first number of photons to one of afirst color and a first gray scale; and mapping said second number ofphotons to one of a second color and a second gray scale.
 55. The methodof claim 53 wherein said generating includes: mapping said first numberof photons to a first shade of gray; and mapping said second number ofphotons to a second shade of gray.
 56. The method of claim 53 whereinthe generating includes generating a radiographic-image-like displayoutput signal.
 57. The method of claim 53 wherein the steps ofdiscretely counting a first number and a second number includesmeasuring a count rate as high as two million photons per second perphoton detector.
 58. The method of claim 53 wherein the step ofirradiating the fast-moving target object includes irradiating with afan beam from a gamma radiation source.
 59. The method of claim 53wherein the step of irradiating the fast-moving target object includesirradiating with a fan beam of 662 keV photons from the radiationsource.
 60. The method of claim 53 wherein the step of irradiating thefast-moving target object includes irradiating with a fan beam from amono-energetic radiation source.
 61. The method of claim 53 wherein thestep of irradiating the fast-moving target object includes irradiatingwith a fan beam from a mono-energetic radiation source.
 62. The methodof claim 53 wherein the step of irradiating the fast-moving targetobject includes irradiating with a fan beam from a near mono-energeticradiation source.
 63. The method of claim 53 wherein the step ofirradiating the fast-moving target object includes irradiating with afan beam from a Cesium-137 radiation source.
 64. The method of claim 53wherein the step of irradiating the fast-moving target object includesirradiating with a fan beam from a Cobalt-60 radiation source.
 65. Themethod of claim 53 wherein the step of irradiating the fast-movingtarget object includes irradiating with a fan beam of photons with highenough energy to penetrate through the fast-moving target object. 66.The method of claim 53 wherein the step of irradiating the fast-movingtarget object includes irradiating with a fan beam of relatively verylow photon intensity for purposes of scanning the fast-moving targetobject.
 67. The method of claim 53 wherein the step of irradiating thefast-moving target object includes irradiating with a fan beam from acurie radiation source having a strength of from 0.3 to 2.0 curies.